One prior art coned disc spring for use as a diaphragm is shown in FIG. 19 which is made of a plate-shaped material and has a circular central portion 10 and its peripheral portion corrugated to form rings P with a wave-shaped cross-section. The wave rings P in the drawings indicate the troughs of the corrugations (to be so understood hereinafter).
However, since this article Dc has its peripheral fixing portion subjected to brazing or the like, it has a high rigidity at this portion. Also it has a high rigidity at its central portion because the radius of curvature is small at this portion. Thus it is less liable to deflect at its peripheral and central portions. As a result, the deflecting force tends to concentrate on the intermediate portion thereof. This will increase the possibility of buckling and cracking owing to metal fatigue if the plate is made of a metal and also the characteristics, specifically the restoring force, of the plate can change during long use.
Typical keyboard switch structures are shown in FIGS. 16-18, but with the cone-shaped spring of the present invention therein. The one shown in FIG. 16 has a contact 42 on the inner bottom surface of a keyboard case 41. If a conventional disk-shaped spring is provided above the contact 42, as a key top 43 is depressed, the spring, which is connected to the key top 43 through a stem 44, will deflect downwards, thus actuating the contact 42. The key top 43 will be returned to its original position by the restoring force of the spring D. It is also known to form part of the contact 42 from the spring D if the spring has conductivity. The key switch shown in FIG. 17 has a coil spring 46 interposed between the keytop 43 and the stem 44 to improve the depressibility. The key switch shown in FIG. 18 has a lever 45 interposed between the disc shaped spring and the stem 44. The abovementioned depressibility refers to the relationship between the depth of depression of the key and the depressing force exerted by a fingertip.
Such conventional disc-shaped spring as used in the above-described switches has an ordinary flat pressure-receiving surface (contact surface with the stem 44). No modifications thereto have been proposed. Their deflectability and restorability after having been deflected depend largely on the properties of their material.
But such an improvement-by-material approach is not only reaching its limit but is also costly.
Also, the keytop 43 is not always depressed at the center of its top surface. A skilled operator who can type at a high speed tends to press the keytop at its corner portion. This has the following effect. By depressing the keytop at its corner, the depressing force is divided into a vertical component and a horizontal component, thus deflecting the spring D with both component forces. This may improve the operability.
But if the spring has a flat pressure-receiving surface, it will offer resistance to the horizontal component but will not deflect. Thus the operability is not good.
If a coned disc spring is used as a diaphragm, it is a common practice to use two annular flanges 22 to grip it therebetween from both sides along its edge, like the flanges 22 as shown on the disc D of the present invention in FIGS. 8(a) and (b).
The flanges 22 are bonded to the edge of the coned disc spring by means of an epoxy or silicone adhesive or by soldering at low temperature (190.degree. to 210.degree. C.).
But with such conventional bonding means, it is necessary to keep the coned disc spring and the flanges fixed in position until the adhesive or the solder hardens. Thus the productivity is bad. Further since adhesives and solders have poor heat resistance, they cannot keep their characteristics for a long period of time but tend to degrade with time.
Further it requires a highly skilled technique to secure such conventional support means to the coned disc spring so that a uniform outward (centrifugal) tension will act on the spring D over the entire area. If the tension is not imparted uniformly, its properties will be badly affected.
FIGS. 14 and 15 show an ordinary pressure detector with a diaphragm according to the present invention. It has a casing 31 formed with a pressure detecting chamber 33 partitioned by a diaphragm D. One compartment 33a in the pressure detecting chamber 33 is filled with a pressure detecting fluid a. In another compartment 33b, there is provided a switch adapted to be activated when the diaphragm D deflects by a predetermined amount.
When such conventional pressure detectors use one having a diaphragm (coned disc spring) D.sub.c as shown in FIG. 19, this diaphragm D.sub.c has a problem that it tends to degrade with time. If it degrades, the detected value of pressure will change. This will lower its reliability.
It is therefore an object of the present invention to provide a coned disc spring and a diaphragm which deflects uniformly in a circumferential direction so that uniform stress will act thereon over the entire area.
It is another object of the present invention to apply the coned disc spring to a diaphragm assembly so that it can maintain a high reliability of detection for many years and to a keyboard switch to improve its operability.
It is still another object of the present invention to provide a method for simplifying the mounting of supporting means of the diaphragm and a method for imparting a uniform centrifugal tension to the diaphragm over the entire area thereof.